1. Field of the Invention
The present invention relates to the processing of dispersions of inorganic fillers in polyester resins and, more particularly, to a method for reducing the times required to accomplish gel and cure of alumina trihydrate filled polyester resins.
2. Description of the Art
Inorganic filler materials, including hydrates such as alumina trihydrate (Al.sub.2 O.sub.3.3H.sub.2 O), are used in large quantities as flame retardant and smoke suppressive fillers for plastics, particularly for thermosetting polyesters. Such materials resist fire and heat and may contain water of hydration which is slowly released therefrom by dehydration in a fire situation. Increased loading of the fillers into the mixture necessarily increases the flame retardant and smoke suppressive qualities of the plastic material.
In preparing polyester-trihydrate dispersions, a polyester resin containing a monomer, such as styrene, is typically mixed with alumina trihydrate in approximately equal quantities. Smaller amounts of fillers or pigments may be added to the mixture. Then a catalyst, such as methyl ethyl ketone peroxide, is added to cause the resin to react with the monomer resulting in the extension and crosslinking of the molecular chains creating a three dimensionally chained structure. The structure is cured to form a rigid, infusible thermosetting material. Curing may be achieved by exposing the structure to radiation, ultraviolet light and heat or by using a room temperature catalyst. Unless the catalyst is used in conjunction with a proper promoter or accelerator, the resin polymerizes very slowly. Therefore, promoters are often used to speed the reaction at room temperature.
The curing of polyester thermosetting polyester resins proceeds in two distinct stages. The first is the formation of a soft gel from a fluid consistency. The time required to reach gelling from the point of adding the catalyst is called the gel time. Soon after the gelation, the second phase of the cure takes place with considerable evolution of heat. The time measured from the moment of catalyst addition to the point of the peak temperature is called peak time or sometimes referred to as cure time. The gel time and the peak time, especially the former, represent two important properties of the curing process. Most of the polyester resins are cured at room temperature. Consequently, a promoter, such as cobalt naphthenate or cobalt octoate, is typically used to speed the curing reaction. Use of more promoter in the formulation of an alumina trihydrate filled polyester resin decreases the time required to cure the polyester resin. However, too much use of the promoter may have adverse effects by measurably shortening the shelf life of the resin, not providing good control for the curing process and creating poor mixing and poor fiberglass wet-out. Not until the resin is gelled can the molded parts be easily handled. It can thus be seen that a long gel time slows down the operation. Accordingly, an improved method for dispersing alumina trihydrate in a polyester resin is desired that will result in shorter gel time and peak time, thereby reducing the time required to accomplish the curing operation without using more promoter in the dispersion.